1. Field of the Invention
This invention relates to methods of acrylating naturals resins. The invention also relates to energy curable resin compositions for printing inks.
2. Description of the Related Art
Resins, derived from natural products for example rosin esters are widely used as printing ink vehicles in flexographic and gravure inks. Maleic modified rosin-esters often exhibit good pigment wetting, gloss retention, color retention and adhesion. These esters are prepared from rosin, maleic anhydride and polyols, as for example glycerol or pentaerythritol. In order to get these materials energy curable, acrylic groups have to be attached to the free hydroxyl groups in the rosin esters. The hydroxyl groups in the resins derived from natural products are often sterically hindered, for example due to the bulky rosin moieties in rosin esters. Therefore, the acrylation with acrylic acid is difficult, since severe reaction conditions are necessary, often causing an undesired polymerization of the acrylic functions at high temperatures, due to their thermal instability. Thus, an introduction of acrylic groups under mild conditions is required.
There are various approaches in literature to introduce resins derived from natural products into energy curable materials. In many publications, the principle is to modify these materials prior to esterification in order to increase the reactivity towards the acrylation agents. Some patents disclose the reaction of a rosin with epoxides, e.g., ethylene oxide and subsequent reaction with acrylic acid. Another patent describes the reaction of fumarated rosin with glycidyl acrylate (JP A-591701650). These products are normally low molecular weight compounds. Yet another publication describes the esterification of fumarated rosin esters with hydroxyethyl acrylate at high temperatures above 200° C. (WO 87/04448, see Examples 3 and 6). At that temperature, there is a high risk of polymerization of acrylated functions even in the presence of an inhibitor.
Japanese Patent Application S58-160313 discloses a photosetting resin that is composed of a disproportionated rosin-modified urethanated unsaturated resin containing as binder components (1) 100 weight parts of a disproportionated rosin with an abietic acid content of 1% (wt %; the same applies hereinafter) or less, (2) 20 to 100 weight parts polyvalent isocyanate, and (3) 10 to 60 weight parts polyfunctional alcohol, having one or more polymerizable unsaturated groups per molecule, and having a hydroxyl value of no more than 80. A reactive diluent such as a (meth)acryloyl compound (which indicates meth and/or acryloyl compounds) or a vinyl group-containing compound can be added.
Japanese Patent Application S58-160313 also discloses a reaction scheme used in forming the photosetting resin by simultaneously or successively sterifying a specified amounts of a disproportionated rosin, a non-resin carboxylic acid and a polyfunctional alcohol. This is followed by a urethanation reaction which is brought about by an isocyanate. Alternatively, an isocyanate is reacted with an hydroxyalkyl acrylate to form an acrylated isocyanate followed by the addition of the disproportionated rosin. The disproportionated rosin referred to herein above is a modified rosin mainly obtained, for example, by reacting rosin with an abietic acid using a catalyst such as iodine, sulfur, or selenium at a temperature of 200° C. or higher, and this modified rosin has better stability because it contains fewer conjugated double bonds. The reason the degree of disproportionation is indicated by the amount of abietic acid contained is that an isomer having conjugated double bonds such as palustric acid or neoabietic acid changes into abietic acid in the course of disproportionation. The disproportionated character in this resin was disclosed to be critical in that its presence increases the storage stability of the photosetting resin without compromising the photocurability, increase gloss due to higher affinity with pigments, increases emulsification resistance by making the resin more hydrophobic, and makes it easier to clean the rollers of a printer due to better solubility in aliphatic hydrocarbons, among other benefits.